Acoustic transducers convert acoustic waves into electrical signals and vice versa. Some common examples include ultrasonic transducers for ultrasound waves which typically have frequencies greater than the human audible limit of approximately 19-20 kHz. Other examples include sonic transducers such as microphones and speakers for audible signals. Those devices that both transmit and receive may also be called acoustic transceivers; many acoustic transducers besides being sensors are indeed transceivers because they can both sense and transmit. These devices work on a principle similar to that of transducers used in radar which evaluate attributes of a target by interpreting the echoes from radio waves. Active acoustic sensors generate acoustic waves and evaluate the echo which is received back by the sensor. These sensors measure the time interval between sending the signal and receiving the echo to determine the distance to an object. Passive acoustic sensors are basically microphones that detect acoustic signals that are present under certain conditions, convert it to an electrical signal, and report it to a computer.
An array of acoustic transducers yields a phased array (PA) acoustic system, where each of the transducers can be operated independently. By varying the pulse timing between the transducers (similar to a radio frequency (RF) antenna phased array), the system can focus the acoustic wave using constructive interference patterns. The system can scan a larger area without having to move or adjust the position of the sensors. Several applications use this technique such as flaw detection in materials (non-destructive testing), medical imaging, ultrasonic sonar for 3D space mapping, haptic feedback using ultrasound waves, microphones and microphone arrays.
However, these systems are typically bulky since acoustic transducers have a relatively large z-height (>>5 mm). Moreover, the assembly of discrete transducers to create a larger phased array increases the cost for a system with a large area (e.g., 10 cm×10 cm) and also may lead to a decrease of the system spatial resolution. MEMS technology used for the creation of acoustic (e.g., sonic or ultrasonic) transducers produces much lower z-height than the above systems. However, manufacturing processes for silicon-based MEMS technology are expensive due to expensive materials and wafer-scale fabrication and can be very challenging or possibly not even feasible over large areas.